(a) Field of the Invention
Broadly speaking, this invention relates to satellite communications systems. More particularly, in a preferred embodiment, this invention relates to a multiple-access, time-division satellite communications system using a common radio frequency for both the up and downlinks to the satellite.
(b) Discussion of the Prior Art
Satellite communications systems have proved to be an effective means of establishing communications between distant geographical areas. The growth of such systems has been limited, however, by the cost of the satellite launch vehicle as well as the cost of the satellite and the ground stations themselves.
In a typical prior art system, the frequency used for the uplink transmission to the satellite differs from the frequency used for the downlink transmission to the ground station, 4 GHZ and 6 GHZ being respectively the transmitting and receiving frequencies most commonly employed for this purpose. This arrangement inevitably complicates the design and implementation of both the satellite and the ground stations, adding to their cost and complexity. It would clearly be preferable to use a common frequency for both the up and downlinks, but heretofore this has not been possible.
Further, while various approaches have been tried to establish multiple-access, satellite communications systems, the fact that different frequencies must be used in the satellite's transmitter and receiver greatly complicates the design of the multiple-access antennas, as well as the switching and multiplexing equipment in the satellite.
Moreover, the coordination of the earth stations, for frequency assignment purposes, has to be considered with respect to two bands; e.g. 4 GHZ and 6 GHZ. This greatly complicates frequency sharing between different satellite communications systems, and between satellite communications systems and terrestrial communications systems because the frequency translation in the satellite systems is relatively fixed, forcing a unique separation on up and down link frequencies.